It can be a useful addition but one thing you need to 100% clear is that you are reading the codes for them and not diagnosing the fault..
that's a very good point, and one that needs to be defined for sure, as I've said, this wouldn't be a "replacement" for the garage, more an additional service at my end for clients, while I'm doing other jobs for them. Frankly I was surprised at the request initially, but I think I lot of people see the workshop activities and think it's "all the same".... I'm very unlikely to take on the car jobs for them, just more give them a technical idea of what they're or what they could be looking at .